PMD is an important factor in the design of state-of-the-art fiber optic transmission systems. The effect of PMD in fiber systems is evident when, after propagating a sufficient distance in the network, one digital pulse may spread in the time domain and become indistinguishable from a nearby pulse. The pulse spreading from PMD can introduce errors into the data transmission, effectively limiting the transmission rate of the pulses or the maximum distance of the concatenated fiber medium.
Fiber manufacturers are therefore interested in providing fibers with low PMD, particularly for products targeted for high data rate, long-haul transmission systems. Unfortunately, measuring PMD directly is an expensive processing step. Accordingly, an easy-to-use, indirect method for identifying fibers with high PMD would be of great value to the industry in that it would reduce measurement (quality control) costs, and therefore overall manufacturing costs, for low PMD optical fibers. Optical time domain reflectometers (OTDRs) have been used to measure a variety of properties of optical fibers. OTDRs operate by sending a short pulse of laser light down an optical waveguide fiber and observing the small fraction of light that is scattered back towards the source. Typical pulsewidths may range from 0.5 meters (5 nanoseconds) to 2000 meters (20 microseconds).
In practice, the fiber under test is connected to the OTDR by a relatively short length of fiber (e.g., a one kilometer length of fiber) known in the art as a "pigtail." The pigtail reduces the deadzone (non-linear region) at the start of the fiber where the OTDR does not provide reliable information. To further improve performance, an index matching oil can be used at the junction between the pigtail and the fiber.
A typical OTDR trace is shown in FIG. 1 where returned power in dBs is plotted along the y-axis and distance down the fiber is plotted along the x-axis. Various features of this trace are identified by the reference numbers 1 through 9, where the number 1 shows the reflection which occurs at the junction between the OTDR and the pigtail, the number 2 shows the trace obtained from the pigtail, the number 3 shows the last point of the pigtail and the first point of fiber under test, the number 4 shows the reflection and associated deadzone produced by the junction between the pigtail and test fiber, the number 5 shows the first point after the near-end deadzone at which trace information can be examined reliably (the "fiber start"), the number 6 shows the fiber trace between the fiber start and the physical end of the fiber (the "fiber end"), the number 7 shows the fiber end, the number 8 shows the reflection which occurs at the fiber end, and the number 9 shows the inherent noise level of the OTDR trace.
Some reports of cyclical patterns in OTDR traces have appeared in the literature. Thus, at the Jan. 24, 1995 TIA 6.6.5 Standards Meeting, Casey Shaar of Photon Kinetics presented a report entitled "Bumpy Fiber Effects." The report describes ripple-like patterns in OTDR traces. The ripples are said to be caused either by polarization effects or by the OTDR source spectrum.
The ripples of this report are different from those of the present invention because, among other things, they have a much shorter period than the cyclical pattern of the present invention (e.g., 200-300 meters versus 2-3 kilometers), are more variable with changes in wavelength (e.g., from 1310 nanometers to 1550 nanometers), and change significantly when observed from different ends of a fiber. Furthermore, the ripples of this reference have a certain "character" in the raw OTDR trace (cycle period, magnitude, shape), and a different character in the mode field diameter (MFD) trace (the ripples may add constructively or destructively). In contrast, the cycles in the raw OTDR trace of the present invention combine in phase in the MFD trace, increasing the amplitude of the cycles but leaving the cycle period and shape unchanged.
Garnham, U.S. Pat. No. 5,518,516, describes ripples in OTDR traces which are said to be caused by helical ridges introduced during the preform laydown process. The patent describes a process for preparing preforms which is said to eliminate such ripples. The ripples which Garnham describes generally extend over the entire length of a blank, whereas the ripples which are the subject of the present invention generally start and stop at different parts of a blank. In practice, ripples of the type described in Garnham have been found not to correlate with elevated levels of PMD.
With regard to the present invention, it is important to note that neither the Photon Kinetics paper nor the Garnham patent contains any suggestion that ripples in OTDR or MFD traces can be used to identify fibers which exhibit elevated levels of PMD.